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The synthesis, characterization, and reactivity of several group 4 metal complexes featuring a central anionic pyrrole moiety connected via CH 2 linkers to two phosphine donors is described. Treatment of [P(NH)P- i Pr] with [MCl 4 (THF) 2 ] (M = Zr, Hf) in the presence of base yields the dimeric complexes [M(PNP i Pr )(μ-Cl)(Cl) 2 ] 2 featuring two bridging chloride ligands. These complexes react with sodium cyclopentadienyl and SiMe 3 I to give the mononuclear complexes [M(PNP i Pr )(η 5 -Cp)(Cl) 2 ] and [M(PNP i Pr )(I) 3 ], respectively. The latter react with MeMgBr to form the trialkyl complexes [M(PNP i Pr )(Me) 3 ]. Upon treatment of [Ti(NMe 2 ) 4 ] with [P(NH)P- i Pr] a complex with the general formula [Ti(PNP i Pr )(NMe 2 ) 3 ] is obtained. DFT calculations revealed that the most stable species is [Ti(κ 1 N - PNP i Pr )(NMe 2 ) 3 ] featuring a κ 1 N -bound PNP ligand. When [P(NH)P- i Pr] is reacted with [Ti(NMe 2 ) 4 ] in CH 2 Cl 2 complex [Ti(PNP i Pr )(Cl) 2 (NMe 2 )] is formed. Treatment of a solution of [P(NH)P- i Pr] and [Zr(NMe 2 ) 4 ] with SiMe 3 Br affords the anionic seven-coordinate tetrabromo complex [Zr(PNP i Pr )(Br) 4 ][H 2 NMe 2 ]. The corresponding hafnium complex [Hf(PNP i Pr )(Br) 4 ][H 2 NEt 2 ] is obtained in similar fashion by utilizing [Hf(NEt 2 ) 4 ] as metal precursor. All complexes are characterized by means of NMR spectroscopy. Representative complexes were also characterized by X-ray crystallography., Supplementary Information: The online version contains supplementary material available at 10.1007/s00706-024-03171-x., (© The Author(s) 2024.)

Treatment of [Co 2 (CO) 8 ] with the ipso -substituted P(C-X)P Y ligands (X = Br, Cl; R =  i Pr, t Bu) bearing Y = NH and CH 2 linkers under solvothermal conditions affords the five-coordinate Co(I) and Co(III) complexes [Co I (PCP Y -R)(CO) 2 ] and [Co III (PCP Y -R)X 2 ]. The later are paramagnetic exhibiting a solution magnetic moment in the range of 3.0-3.3 μ B which is consistent with a d 6 intermediate spin system corresponding to two unpaired electrons. In the case of P(C-X)P Y ligands (X = Br, Cl; R =  t Bu; Y = NH) the formation of the square planar Co(II) complex [Co(PCP NH - t Bu)X] was favored. This complex gives rise to a magnetic moment of 1.8 μ B being consistent with a d 7 low spin system corresponding to one unpaired electron. All complexes are characterized by means of spectroscopic techniques (NMR, IR), HR-MS. Representative complexes were also characterized by X-ray crystallography., Supplementary Information: The online version contains supplementary material available at 10.1007/s00706-023-03123-x., (© The Author(s) 2023.)

The synthesis, characterization, and reactivity of several new Cr(II) and Cr(III) complexes featuring an NCN pincer ligand with an arene backbone connected to amine donors NEt 2 and N i Pr 2 via CH 2 -linkers is described. Reacting the in situ lithiated ligand precursor N(C-Br)N CH2 -Et with [CrCl 3 (THF) 3 ] resulted in the formation of the Cr(III) complex trans -[Cr(κ 3 NCN -NCN CH2 -Et)(Cl) 2 (THF)]. Upon reaction of lithiated N(C-Br)N CH2 - i Pr with a suspension of anhydrous CrCl 2 , the Cr(II) complex [Cr(κ 2 NC -NCN CH2 - i Pr) 2 ] is formed featuring two NCN ligands bound in κ 2 NC -fashion. In contrast, when lithiated N(C-Br)N CH2 - i Pr is reacted with a homogeneous solution of anhydrous Cr X 2 ( X  = Cl, Br), complexes [Cr(κ 3 NCN- NCN CH2 - i Pr)X] are obtained. Treatment of [Cr(κ 3 NCN- NCN CH2 - i Pr)Cl] with 1 equiv of PhCH 2 MgCl and LiCH 2 SiMe 3 afforded the alkyl complexes [Cr(κ 3 NCN -NCN CH2 - i Pr)(CH 2 Ph)] and [Cr(κ 3 NCN -NCN CH2 - i Pr)(CH 2 SiMe 3 )]. All Cr(II) complexes exhibit effective magnetic moments in the range of 4.7-4.9  µ B which is indicative for d 4 high spin systems. If a solution of lithiated N(C-Br)N CH2 - i Pr is treated with CrCl 2 , followed by addition of an excess of Na[HB(Et) 3 ], the dimeric complex [Cr(κ 2 NC -NCN CH2 - i Pr)(μ 2 -H)] 2 is obtained bearing two bridging hydride ligands. [Cr(κ 3 NCN -NCN CH2 - i Pr)(CH 2 SiMe 3 )] turned out to be catalytically active for the hydrosilylation of ketones at room temperature with a catalyst loading of 1 mol%. X-ray structures of all complexes are presented., Supplementary Information: The online version contains supplementary material available at 10.1007/s00706-023-03128-6., (© The Author(s) 2023.)

The synthesis and characterization of two Co(II) complexes stabilized by a tridentate SCS pincer ligand are described. Paramagnetic [Co(κ 3 -SCS CH2 -Et) 2 ] and [Co(κ 3 -SCS CH2 - t Bu)(κ 2 -SCS CH2 - t Bu)] were obtained via transmetalation protocol from CoBr 2 and S(C-Br)S CH2 -R (R = Et, t Bu). Oxidation of the latter with [Cp 2 Fe]PF 6 affords the diamagnetic 18 VE complex [Co(κ 3 -SCS CH2 - t Bu) 2 ]PF 6 . X-ray structures and DFT calculations are presented., Supplementary Information: The online version contains supplementary material available at 10.1007/s00706-022-02949-1., (© The Author(s) 2022.)

Abstract: An iron(II) hydride PNP pincer complex is applied as catalyst for the chemoselective transfer hydrogenation of aldehydes using an aqueous solution of sodium formate as hydrogen source. A variety of aromatic, heteroaromatic, and aliphatic aldehydes could be reduced to the corresponding alcohols in good to excellent yields with a catalyst loading of 1.0 mol% at 80 °C and 1 h reaction time. If present, C-C double bonds remained unaffected in course of the reaction, even when they are conjugated to the carbonyl group of the aldehyde. The catalyst's lifetime and activity could be improved when the reactions were conducted in an ionic liquid-based micro emulsion.

Abstract: A series of neutral bis- and cationic tris-carbonyl complexes of the types cis -[M(κ 3 P,N,P -PNP)(CO) 2 Y] and [M(κ 3 P,N,P -PNP)(CO) 3 ] + was prepared by reacting [M(CO) 5 Y] (M = Mn, Re; Y = Cl or Br) with PNP pincer ligands derived from the 2,6-diaminopyridine, 2,6-dihydroxypyridine, and 2,6-lutidine scaffolds. With the most bulky ligand PNP NH - t Bu, the cationic square-pyramidal 16e bis-carbonyl complex [Mn(PNP NH - t Bu)(CO) 2 ] + was obtained. In contrast, in the case of rhenium, the 18e complex [Re(PNP NH - t Bu)(CO) 3 ] + was formed. The dissociation of CO was studied by means of DFT calculation revealing in agreement with experimental findings that CO release from [M(κ 3 P,N,P -PNP)(CO) 3 ] + is in general endergonic, while for [Mn(κ 3 P,N,P -PNP NH - t Bu)(CO) 3 ] + , this process is thermodynamically favored. X-ray structures of representative complexes are provided.

Abstract: The new chiral PNP pincer ligand N 2 , N 6 -bis((3a R , 8a R )-2,2-dimethyl-4,4,8,8-tetraphenyltetrahydro[1,3]dioxolo[4,5- e ][1,3,2]dioxaphosphepin-6-yl)pyridine-2,6-diamine (PNP-TADDOL) was synthesized in 80% isolated yield. Complexes of the type [M(PNP-TADDOL)(CO) 3 ] (M = Cr, Mo, and W) were prepared via a solvothermal approach. This methodology constitutes a fast, simple, and practical synthetic method to obtain complexes of that type in high isolated yields. The X-ray structure of the molybdenum complex is presented.

Abstract: Kumada-Corriu and Negishi cross-coupling reactions, catalyzed efficiently by a Ni(II) PNP pincer complex containing a triazine backbone, are described. The catalyst is able to react with both activated and inactivated aryl halides including chlorides as well as phenol derivatives such as tosylates and mesylates to give the corresponding cross-coupling products in good to excellent isolated yields. A high diversity of substrates was tested under moderate conditions for both types of reactions.

Abstract: Several new monophosphorylated 2,6-diaminopyridine ligands bearing P i Pr 2 and P t Bu 2 units (PN NH2 - i Pr, PN NH2 - t Bu, PN NHMe - i Pr, and PN NHEt - i Pr) are prepared by treatment of the respective 2,6-diaminopyridines with the chlorophosphines P i Pr 2 Cl and P t Bu 2 Cl in the presence of a base. Treatment of anhydrous FeCl 2 with 1 equiv of these afforded the tetracoordinated coordinatively unsaturated 14e - complexes [Fe(κ 2 P,N -PN NH2 - i Pr)Cl 2 ] and [Fe(κ 2 P,N -PN NH2 - t Bu)Cl 2 ], while with PN NHMe - i Pr and PN NHEt - i Pr a phosphine transfer reaction of a second PN ligand took place to yield the known PNP pincer complexes [Fe(κ 3 P,N,P -PNP Me - i Pr)Cl 2 ] and [Fe(κ 3 P,N,P -PNP Et - i Pr)Cl 2 ]. The four-coordinate complexes [Fe(κ 2 P,N -PN NH2 - i Pr)Cl 2 ] and [Fe(κ 2 P,N -PN NH2 - t Bu)Cl 2 ] did not react with CO and the formation of iron PNC pincer complexes was not observed. The reason for the reluctance to add CO was investigated in detail by DFT calculations.

Abstract: In the present work, we have prepared a series of octahedral Fe(II) complexes of the type trans -[Fe(PNP)(CO) 2 Cl] + -PNP are tridentate pincer-type ligands based on 2,6-diaminopyridine. These complexes are formed irrespective of the size of the substituents at the phosphorus sites and whether cis -[Fe(PNP)(Cl 2 )(CO)] or trans -[Fe(PNP)(Cl 2 )(CO)] are reacted with CO in the presence of 1 equiv of silver salts. X-ray structures of representative complexes are presented. Based on simple bonding considerations the selective formation of trans -dicarbonyl Fe(II) complexes is unexpected. In fact, DFT calculations confirm that trans -dicarbonyl complexes are indeed thermodynamically disfavored over the respective cis -dicarbonyl compounds, but are favored for kinetic reasons.

Abstract: A new asymmetric chiral PNP ligand based on the 2,6-diaminopyridine scaffold featuring a R -BINEPINE moiety was prepared. Treatment of anhydrous FeX 2 (X = Cl, Br) with 1 equiv of PNP- i Pr,BIN at room temperature afforded the coordinatively unsaturated paramagnetic complexes [Fe(PNP- i Pr,BIN)X 2 ]. The structure of [Fe(PNP- i Pr,BIN)Cl 2 ] is described. Both complexes react readily with the strong π-acceptor ligand CO in solution to afford selectively the diamagnetic complexes trans -[Fe(PNP- i Pr,BIN)(CO)X 2 ] in quantitative yield. Due the lability of the CO ligand, these complexes are only stable under a CO atmosphere and isolation in pure form was not possible. The preparation of the carbonyl hydride complex [Fe(PNP- i Pr,BIN)(H)(CO)Br] was achieved albeit in low yields via a one pot procedure by treatment of [Fe(PNP- i Pr,BINEP)Br 2 ] with CO and subsequent reaction with Na[HBEt 3 ]. This complex was obtained as an inseparable mixture of two diastereomers in a ca. 1:1 ratio and was tested as catalyst for the hydrogenation of ketones. The catalyst showed acceptable activity under mild conditions (5 bar H 2 , room temperature) with yields up to >99 % within 18 h.

Abstract: The new PSP pincer ligand 4,6-bis(diphenylphosphinomethyl)dibenzothiophene (PSP Ph ) was prepared in 89 % yield. With this ligand, a solvothermal synthesis of a Cr complex of the type [Cr(κ 3 P,S,P -PSP)(CO) 3 ] is described. The X-ray structure of this compound is presented. We demonstrate that the solvothermal synthesis technique provides a powerful, simple, and practical synthetic method resulting in a high isolated yield in a short reaction time.

Abstract: A Ru-catalyzed mono-arylation in α-position of saturated cyclic amines is reported employing a C-H activation protocol. Substitution of the pyridine directing group with a bulky group, e.g., trifluoromethyl in the 3-position, proved to be crucial to avoid bis-arylation. This highly selective transformation can be performed with different amines and arylboronate esters. Additionally, the directing group can be cleaved, taking advantage of an unprecedented detrifluoromethylation reaction.